The present invention relates to a vehicle, and in particular to an all wheel drive vehicle having a passive coupling.
Vehicles currently have stability control systems that prevent lateral instability of the vehicle. Heretofore, stability control systems have included a brake based control system similar to an anti-lock brake control system. The brake based stability control system makes a stability correction by applying one or more of the brakes independent of a driver of the vehicle during an oversteer or understeer condition. Therefore, the stability control system will drive one or more of the wheels to deep slip, thereby generating a counter yaw on the vehicle to counter the oversteer or understeer condition. However, when the vehicle is an all wheel drive vehicle having a passive coupling between a primary driveshaft and a secondary driveshaft, the effectiveness of the stability control correction may be reduced by a negative torque transferred from one driveshaft to the other driveshaft due to the inability of the vehicle to completely disconnect the secondary driveshaft from the primary driveshaft.
Accordingly, an apparatus solving the aforementioned disadvantages and having the aforementioned advantages is desired.
One aspect of the present invention is to provide a method for overcoming negative torque transfer in a passive coupling in a vehicle having all wheel drive. The vehicle includes a front driveshaft and a rear driveshaft, with the passive coupling connecting the front driveshaft and the rear driveshaft. The vehicle also includes a transmission operatively connected to the passive coupling. The method includes determining negative torque transfer in the passive coupling and requesting the vehicle to increase transmission output equal to the negative torque transfer.
Another aspect of the present invention is to provide a vehicle system for overcoming negative torque transfer in a vehicle having all wheel drive. The vehicle system includes a front driveshaft, a rear driveshaft, a passive coupling connecting the front driveshaft and the rear driveshaft, a passive torque control system, and a transmission operatively connected to the passive coupling. The passive torque control system calculates a front driveshaft rotational speed of the front driveshaft and a rear driveshaft rotational speed of the rear driveshaft, and calculates a speed difference across the passive coupling by subtracting the front driveshaft rotational speed from the rear driveshaft rotational speed. The passive torque control system also determines negative torque transfer in the passive coupling corresponding to the speed difference and requests an output from the transmission equal to the negative torque transfer.
Yet another aspect of the present invention is to provide a method of overcoming negative viscous torque transfer in a passive coupling in a vehicle having all wheel drive. The vehicle includes a front axle having a first front wheel and a second front wheel, a front driveshaft connected to the front axle, a rear axle having a first rear wheel and a second rear wheel, and a rear driveshaft connected to the rear axle. The vehicle also has the passive viscous coupling connecting the front driveshaft and the rear driveshaft. The vehicle further includes a transmission operatively connected to the passive coupling. The method comprises measuring a first front rotational speed of the first front wheel, measuring a second front rotational speed of the second front wheel, measuring a first rear rotational speed of the first rear wheel and measuring a second rear rotational speed of the second rear wheel. The method also comprises calculating a front driveshaft rotational speed using the formula ((f1+f2)/2)*rf, wherein f1 equals the first front rotational speed, f2 equals the second front rotational speed, and rf equals the axle ratio of the front driveshaft over the first front wheel or the second front wheel. The method further comprises calculating a rear driveshaft rotational speed using the formula ((r1+r2)/2)*rr, wherein r1 equals the first rear rotational speed, r2 equals the second rear rotational speed, and rf equals the axle ratio of the rear driveshaft over the first rear wheel or the second rear wheel. The method further includes the step of calculating a speed difference across the passive viscous coupling by subtracting the front driveshaft rotational speed from the rear driveshaft rotational speed. The method also includes the steps of determining negative viscous torque transfer in the passive coupling corresponding to the speed difference and requesting the vehicle to increase transmission output equal to the negative viscous torque transfer.
Accordingly, the vehicle system provides a torque output from the engine and the transmission that will offset the negative torque transfer from a stability maneuver to allow the stability maneuver to effectively stabilize the vehicle. The vehicle system is easy to implement, capable of a long operable life, and particularly adapted for the proposed use.
These and other features, advantages, and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.